Pollutants produced by combustion include oxides of nitrogen which are more commonly referred to as NO.sub.x (where x is an integer which represents the number of oxygen atoms in the molecule). Such oxides include NO and NO.sub.2. In the combustion process NO.sub.x is formed by air (a gas containing nitrogen and oxygen) being subjected to high temperatures for a period of time. Recent studies have been made on lower 90% distillation temperature gasoline which show that faster burning gasoline (lower 90% distillation temperature) comes up to high temperature more rapidly increasing the time the nitrogen and oxygen in the air are exposed to high temperature thereby causing an increase in NO.sub.x (see the FIGURE). This type of fuel is described in U.S. Pat. No. 5,015,356 which is incorporated herein by reference.
Gasolines now used as fuel in current spark ignited internal combustion engines require octane numbers (R+M)/2 falling almost entirely within the range of 84-94. Some engines require a higher octane gasoline than others depending on their compression ratio or carbon deposit buildup (age) in order to avoid or reduce engine "knock" or to improve fuel combustion efficiency.
Lower octane gasolines have been used in the past in low compression engines but were abandoned in common practice because of poor efficiency and power output. It would be highly desirable if lower octane gasolines could be efficiently used in currently available engines especially if their use resulted in reduction of NO.sub.x pollution.